The present invention generally relates to methods for fabricating composite structures. More specifically, this invention relates to a method of fabricating a composite article to have an integral composite secondary structure, for example, an integral flange of a composite casing for a turbomachine.
Composite materials generally comprise a fibrous reinforcement material embedded in a matrix material, which in the case of a polymer composite material is a polymer material (polymer matrix composite, or PMC). The fibrous reinforcement of a composite material serves as the secondary constituent of the material, while the matrix material protects the reinforcement, maintains the orientation of its fibers, and serves to dissipate loads to the reinforcement.
Composite materials have become increasingly popular for use in a variety of aerospace applications because of their durability and relatively light weight. Particular but nonlimiting examples include the use of PMC materials for fan casings for aircraft gas turbine engines. Although composite materials can provide superior strength and weight properties, designing flanges and other secondary features on structures fabricated from composite materials poses a challenge. As an example, composite structures having laminate constructions that contain continuous reinforcement materials are capable of exhibiting superior in-plane strength due to the presence of the continuous reinforcing fibers. As used herein, continuous reinforcement materials refer to continuous fibers or fiber bundles (tows) that are typically oriented to have a specific orientation (unidirectional) within a matrix material of a composite, for example, parallel to the load direction on the composite, in contrast to discontinuous fiber reinforcement materials made up of short fibers that are typically randomly dispersed in a matrix material. While composite structures containing continuous fiber reinforcement are capable of exhibiting superior in-plane strength, flanges and other secondary structures that extend out of the plane of the continuous reinforcing fibers lack continuous fibers at their points of attachment, or joints, with the primary composite structure. The lack of continuous fibers, as well as the likelihood of significant out-of-plane loads created by attachments to the flanges, may result in relatively weaker attachment joints that are susceptible to damage from increased stresses. Though it is possible to separately fabricate a flange and then attach the flange to a primary composite structure with a supplemental reinforcement structure, for example, additional fibers or metal brackets, the weight-saving benefits possible with the use of composite materials can be significantly reduced as a result.
In the case of composite casings of gas turbine engines, integral flanges constructed of fiber preform designs have been proposed to address structural weaknesses at the point of attachment. However, such fiber preform design options tend to be quite limited. Plies used in the construction of composite casings and their integral flanges are typically woven or braided preforms that limit fiber orientation, with the result that stiffness and strength cannot be readily optimized. The degree to which interleaving between fiber tows within the casing and within the flange body is also typically limited, resulting in limited resistance to delamination.
Accordingly, there is a need for improved techniques by which a composite structure can be fabricated to have an integral composite flange or other secondary composite structure with continuous fibers at points of attachment therebetween.